1. The present invention relates to absorbent, acoustic passive antennas that can be used to receive the acoustic signals in order to convert them into electrical signals to be processed and, at the same time, to absorb these very same acoustic signals, especially when they come from an active detection sonar. It can be applied more particularly to antennas having large surface areas, designed to be fixed with a shape matching that of the hull of a ship.
2. Description of the Prior Art
For some years now, acoustic antenna structures have been developed with large dimensions, for example several square meters. These antennas are called surface antennas. These antennas essentially use, as sensors, piezoelectric poller elements, generally of the PVF2 type. In view of these dimensions, it becomes necessary to fix the antennas to the hull of the boat bearing the sonar to which they are linked. These antennas are shaped so that they match the shape of the hull to which they are attached. Under these conditions, in order to ensure the performance characteristics of the system, it is essential to have control over three essential criteria:
a hydrodynamic criterion to avoid lowering the performance characteristics of the carrier ship, this being achieved precisely by making an antenna with a matching shape; PA1 an acoustic barrier criterion to obtain an efficient rejection of noises coming from the rear of the antenna, hence from the carrier boat, to avoid masking the useful signals; PA1 a criterion of anechoic quality to absorb the acoustic signals at the frequencies used by underwater weapons such as torpedoes by using precisely the fact that the surface area of the hull covered by the antennas is great.
It will be noted however that these criteria may be met with standard localized sensors, and not by the use of a piezo-electrical polymer, when the frequencies used exceed one or two kHz, provided however that the matching shape of the antenna is maintained. This is relatively difficult with a large number of small sensors, made of ceramic for example.
In a French patent application No. 92 06274 filed on 22nd May 1982 and delivered on 26th Nov. 1993 under U.S. Pat. No. 2,691,569, (corresponding to U.S. Pat. No. 5,517,467) the present applicant has described a antenna for submarines that has a matching shape and that essentially meets the hydrodynamic criterion described here above.
There also exist known "soft" acoustic reflectors designed to be placed in the rear of the cylindrical antennas, which are themselves located in the front of surface boats. Soft reflectors such as these are described especially in the French patent application No. 83 00753 filed by the applicant on 19th Jan. 1983 and delivered under U.S. Pat. No. 2,539,541. It is also the practice sometimes, in the place of these soft reflectors, to use hard type reflectors, namely high impedance reflectors.
The antennas that best meet the above-defined criteria are essentially devices designed to act as side antennas for high-tonnage antennas, the structure of which is defined in a sectional view in the appended FIG. 1.
The antenna is placed on the thick hull 101 of a submarine and comes into contact with water by means of intermediate apertures made in the thin hull 102 of this submarine.
The hydrophones 103, embedded in a protective layer, are fixed to the surface of a metal plate 104 that plays the role of a hard reflector and is flush with the thin hull 102.
This plate 104 is itself borne by compliant supports 105 which are furthermore fixed to the surface of the thick hull 101. These compliant supports demarcate an internal space 106 that has reflective and absorbent elements as described here below. The rest of this internal space 106 is filled with water.
In the cavity 106, there are therefore soft type absorbent reflectors 107 fixed to the lower face of the metal plate 104, and absorbent reflectors 108 formed by compliant tubes embedded in a soft matrix and fixed to the external surface of the thick hull.
The heterogeneity of such a structure implies that it is complicated to build and difficult to position on rounded hulls. Furthermore, the thickness of this antenna conventionally reaches 300 mm, which is considerable. These difficulties result in a high cost that becomes prohibitive.